# Tag Info

20

Kirchhoff's law is only valid for objects in radiative equilibrium. The emissivity and absorptivity of a material are the same for a given wavelength, but can vary dramatically for different wavelengths. The radiators on a spacecraft are not in radiative equilibrium, since they lose heat to radiation. They emit heat in the longwave infrared spectrum, but ...

13

The spacecraft's own truss and elements are thermally conductive, so there's no need for hot air heat distribution. Air (or gas) is actually not such a good heat conductor due to its low density, unless it's highly compressed, at which point you risk mechanical damage to parts it would be heating up. It would also unnecessarily add mass to the spacecraft and ...

12

Note that I am answering only the question in the title, not most of the body of the question. Yes you can run a steam engine on the Moon if you have a source of energy and a way of dissipating waste heat. Steam engines do not boil their water in air: they boil it in boilers which contain water and steam: there is no air in the boiler of a steam engine. ...

12

No: The heat produced by atmospheric reentry isn't a happy side effect of returning to the earth, it's a byproduct of the fact that your satellite/orbiter has enough kinetic energy to be circling the earth every 90 minutes and you want it to stop doing that and come down. To have something you've made for the purpose of harvesting energy reenter the ...

11

Yes, in principle to both questions, why not? However we can calculate the maximum energy you could get from the generator. The vapour pressure of $CO_2$ at 0C is around 4MPa, so in a perfect world, you get a volume expansion of about $4 MPa/600Pa$ or 6400. The formula for the work per mole done by isothermal expansion of a gas is RT\mathop{\mathrm{ln}}...

8

There's no fundamental physical limitation, but there's certainly a practical one. The rocket equation is $\Delta V = V_{Exhaust} \ln(M_{Total}/M_{Dry})$. The exhaust velocity of a typical chemical rocket is around 2500 m/s to 4500 m/s, the exhaust velocity of a steam rocket is only 500 m/s or so. You need a delta-v of around 2 km/s to reach the Karman line....

7

No, it's much too slow for that. The Parker Solar Probe reaches (or at least approaches) thermal equilibrium on its perihelion passes; your hand passing briefly through a flame does not.

6

This paper CFD SIMULATION OF A LIQUID ROCKET PROPELLANT (LH2 /LOx) COMBUSTION CHAMBER claims that the chamber properties are essentially constant until the constriction starts towards the throat.

6

Frank O'Brien in his book "The Apollo Guidance Computer: Architecture and Operation", on p. 229 concludes that effect from sublimator vapour thrusting could've been measurable: A quite real source [of unaccounted in state vector translation] arises with the exhaust from the LM sublimator, used to cool the vehicle's electronics. Although producing the ...

6

Bernoulli's principle only applies to incompressible flow. Once you go compressible, especially when you go supersonic, things become counterintuitive. This design will limit you more or less to an exhaust velocity just barely above Mach 1.

6

Reentry heat is not some magically created energy - it's all dissipation of energy that was painstakingly pumped into the orbital vehicle on ascent, so you will never break even with what you spent on propelling it to the orbit (plus propelling its second stage and its fuel to suborbital speed, plus propelling that fuel to nearly orbital speed, plus all that ...

6

The ability of a metal in the liquid state to block radiation is quite similar to the same metal in the solid state. Ability to block alpha, beta, gamma, and x-ray radiation typically scales with the electron density of the materials. Neutron radiation is more complicated. Metals with high atomic numbers have high electron densities and typically maintain ...

5

The question has been evolving. I've addressed the original: Are there any metals that has a high Armstrong limit? I've never heard of a liquid metal that boils at 20° C or 37° C in a vacuum. Metallic hydrogen might be suggested but that's not a liquid at atmospheric pressure. So I think the answer is pretty much All of them! Things like mercury or ...

5

This formula assumes a constant gravitational acceleration over the whole height of the gas column - a reasonable assumption for Earth, as the atmosphere is thin compared to to the size of the planet. For a simple argument, if you assumed composition is the same, it's clear that the altitude change needed for pressure to change by a certain factor is less ...

5

From this page, the defining feature of a self-pressurizing liquid (also referred to as "Vapor Pressurization", or "VaPak") is that it has a high vapor pressure. Specifically for an oxidizer: Nitrous oxide (N2O) is the most promising oxidizer that can self-pressurize because it is relatively energetic and its vapor pressure is ...

5

If you are planning to boil water at low pressure, it will happen at low temperature, which means you need an even lower temperature to condense the spent steam back to water. An efficient heat engine will operate across a significant temperature difference - the bigger the better. Dissipating heat in space can only be done by radiation; the higher the ...

4

A Detra-Kemp-Riddell model for stagnation heating. Detra, R. W.; Kemp, N. H.; and Riddell, F. R.: Addendum to "Heat Transfer to Satellite Vehicles Re-entering the Atmosphere." Jet Propulsion, vol. 27, no. 12, Dec. 1957, pp. 1256-1257. published online Equation 32 (page 20) and Ref 6 in NASA TM X-2058 A General Transient Heat-Transfer Computer Program for ...

4

Liquid hydrogen will boil off in the tank until the pressure reaches equilibrium.

4

For a spherically symmetric mass distribution in hydrostatic equilibrium: ${dP\over dr}=-g\rho$ where $P$ is the pressure, $r$ is the radius, $g$ is the gravitational acceleration as a function of $r$, and $\rho$ is the density of the gas as a function of $r$. Then you integrate up or down from some known conditions. $g$ as a function of $r$ is ...

4

At Parker Solar Probe’s closest approach to the Sun, temperatures on the heat shield will reach nearly 2,500 degrees Fahrenheit, but the spacecraft and its instruments will be kept at a relatively comfortable temperature of about 85 degrees Fahrenheit. Source is here, admittedly it's far shorter than I had hoped. In addition to the quote stating the ...

3

There has been some research on flames in 0.38g (using parabolic flights). There are no usable photos in that paper, but: Cool flames at 0.3g appear qualitatively similar to those at 1g, yet those at μg are radially (presumably spherically) symmetric. Which is more or less as expected. When you have gravity, you get convection (hot air rising). The ...

3

I took it to be a heat shield, like in another, recent question. In that case multiple panels work better than one, and probably foamed metal is better than solid. It will still heat up, even if more slowly, so you'll still need a radiator on the back end. Even the Space Shuttle and ISS, orbiting Earth, needed/need radiators, probably anything manned in ...

3

It's a decent approximation. The effect of convective heat transfer on the flow is very slight in practical nozzles, since its influence extends only so far as the thermal boundary layer thickness, and this is of the order of the velocity boundary thickness (for typical gases). Radiant heat transfer is also of little importance to propellant ...

3

When designing a rocket engine you need to design the turbo pumps for the propellants and the combustion chamber. To design a turbopump you need to know its output pressure and the volume flow rate. To determine the output pressure you need to know the chamber pressure. To design the walls of the combustion chamber you need to know the chamber pressure to ...

3

The chemistry side of things is usually like this: Make a qualified guess for what exhaust products are formed by the reaction. Calculate the energy freed from that reaction (by the help of delta-h tables). Use the energy freed to find the chamber conditions (temperature/pressure) Look up chemical equilibriums for those chamber conditions to make a better ...

3

The specific impulse will vary with the exact design of the engine (engine cycle, mixture ratio, chamber pressure, nozzle size, and other thermodynamic features), but there's a table on the Wikipedia liquid rocket propellant page giving typical specific impulse (expressed as exhaust velocity in m/s) for various propellant combinations.

3

My question is specifically aimed at the the thermodynamics of boiling water on the moon in the absence of air. If the steam generator were put on the dark side, or in a cave anywhere, the temperature gradiant achievable would be awesome. And in the absence of air, the boiling point would be a lot lower (20degrees C?) In a conventional steam power plant on ...

3

Note that nuclear power and steam are not alternatives to each other. Nuclear power is source of heat. Steam turbine and steam engine are ways turn heat into motion and energy. Most of nuclear power plants still nowadays boil water to produce steam which is then ran through steam turbine. Steam engines are not used because of their lower efficiency. If you ...

2

Universal gas constant R' is correct. Sutton uses $C_p$ for molar specific heat and $c_p$ for mass specific heat. See table of symbols at end of chapter (I have the 7th ed.).

1

"pressure drops in the constrictions" might be misleading. The pressure drops over the constriction. In other words the pressure is higher upstream of the constriction. The peak pressures are in the combustion chamber, which is upstream of the constriction. All this tells you is the the more the constriction, the greater the difference in pressure between ...

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